The poor machinability is a common problem of Engine Castings in China, and it is also the biggest gap between domestic castings and foreign castings. Even though the chemical composition, matrix microstructure and hardness of domestic castings and imported KD castings are similar, the machinability of domestic castings is still far less than that of imported KD castings, and sometimes the consumption of cutting tools is more than double
1.1 effect of trace elements from raw materials
a. The content of Microelements in iron exceeds the standard, such as Ti, V, Pb, be, B, etc. when the content of these microelements is high, some of them are in the form of free carbide, nitride and other hard points (titanium carbide, titanium nitride, etc.), some of them increase the number of hard phase sorbite obviously (such as V, etc.)
b. Scrap iron is mixed with alloy steel (such as Ti, V, etc.), or scrap iron with coating is used. Such as Pb plating waste steel plate.
C. Some elements (such as Pb, be) increase the tendency of white mouth.
1.2 improper smelting process, such as smelting in the electric furnace for a long time, the tendency of molten iron to white mouth is increased
1.3 improper inoculation process, i.e. the inoculant or inoculation process selected fails to eliminate the sensitivity of the casting section, especially the microstructure hard phase at 5mm thin wall
2.1 select appropriate pig iron, control the content of trace elements in pig iron, Ti <. 05%, V ≤ 0.01%, adopt low carbon steel scrap, and forbid to mix alloy steel in scrap
2.2 avoid excessive and unevenly distributed hard phase microstructure in the process of alloying. Generally, in order to ensure good casting performance and achieve high mechanical properties, high carbon equivalent is used with alloying measures. The purpose of alloying is to increase the pearlite volume, refine and stabilize the pearlite, but avoid the tendency of white mouth, avoid segregation, and avoid hard phase microstructure When the structure appears, the alloying elements should be selected and combined reasonably, and the best way is inoculation
2.3 it is very important to improve the cutting performance by adopting effective inoculation process. Generally, the inoculant containing Ca and Ba is better than the traditional inoculant of 75sife. Secondly, flow inoculation process is adopted. Such inoculation process can obtain uniform microstructure and uniform microhardness, especially for the cylinder block (cylinder head) castings with large wall thickness difference, the microstructure and property at the place with the minimum wall thickness of 5mm Can be more even
The above is a superficial analysis of the common and common casting defects in the production of gray cast iron cylinder block (cylinder head) castings of small and medium passenger car engines and their countermeasures. Due to the technical progress, some of them are not often seen, and some of them are not Casting defects, such as mechanical damage, size deviation and sand, which often occur or account for a small proportion, are not involved here